(1) Field of the Invention
This invention relates to a high-efficiency electrostatic filter. More particularly, the invention relates to an electrostatic air filter device designed to be used for cleaning air in a room, which device has a high dust collecting efficiency and a long service life.
(2) Description of the Prior Art
The so-called HEPA filters are generally high in gas collecting efficiency, but they tend to suffer a high head loss when a dust-laden gas is passed therethrough.
In these filters, therefore, if the filter mesh is widened in order to reduce the head loss, there results a reduction of the duct collecting efficiency, and when it is attempted to decrease the head loss by reducing the feed gas rate, the filter size is inevitably enlarged. These filters also had the disadvantages in that the head loss increases sharply with fill-up of the filter meshes and that the service life is short. Therefore, it has been generally attempted to extend the filter life by provising a suitable pre-filter in front of the main filter.
The present inventors had previously worked out a high-efficiency electrostatic filter device having a far higher dust collecting efficiency and a longer service life than the conventional filters, said filter device comprising, as shown in FIGS. 1 and 2 of the accompanying drawings, a charging section 4 consisting of a frame 1 allowing passage of a dust-laden gas, a plurality of grounded parallel plate electrodes 2 housed and fixed in said frame 1 in parallel relation to the direction of gas flow and a plurality of discharging wires 3 provided to extend in the spaces between every adjoining plate electrodes 2 for applying a high voltage, and a dust collecting section 8 comprising a frame 5 allowing passage of the dust-laden gas which has passed the charging section 4, a filter medium 6 housed in said frame 5 and arranged meandering in the direction orthogonal to the gas flow, that is, in the direction intercepting the gas stream, and a plurality of electroconductive separator electrodes 7 allowing passage of the dust-laden gas, said separator electrodes 7 being inserted between every adjoining parts of the meandering filter medium 6 from both upstream and downstream sides of said gas flow, with a high voltage applied between the separator electrodes on the upstream side and those on the downstream side, whereby a substantial portion of the dust particles preliminarily charged in the charging section 4 are attracted to the surfaces of the respective separator electrodes on the upstream side of the dust collecting section 8, letting said dust particles lose their charges and be captured in said section 8 to thereby prevent clogging of the filter medium 6 with the dust particles to a maximal degree.
In said high-efficiency electrostatic filter device, when an electric current is supplied directly to the separator electrodes on both upstream and downstream sides, should electric discharge occur even at a part of a certain separator electrode, the charges of all of the separator electrodes move to the discharging part to increase the discharge energy with the result that there takes place temporary drop of separator voltage to cause a temporary reduction of dust collecting efficiency or even a damage to the filter medium 6 such as formation of holes in the filter medium.
Also, in said high-efficiency electrostatic filter, the spark voltage is decided by the thickness t of the filter medium 6 and the voltage applied between the separator electrodes independently of the space or distance h between the adjoining parts of the filter medium 6 gapped by the respective separator electrodes 7, 7 as shown in FIG. 3. The fact is to be also noted that when the ambient humidity or temperature is high, the filter medium 6 tends to be weakened in its electric insulation to cause an increase of leak current and/or a drop of power source voltage, resulting in a reduced dust collecting efficiency. Such tendency is conspicuous particularly when glass fiber is used as filter medium.